Conformable acoustic coupler

ABSTRACT

A conformable acoustic coupler for ultrasonically testing an article. The upler has an ultrasonic transducer with an active surface which transmits ultrasonic energy pulses to an article to be tested. A sensor receives and measures an ultrasonic echo from the tested article. The article to be tested is acoustically mated to the active surface by a medium of a gelatinous material layer on the active surface and a flexible film on the gelatinous material layer. The combination of the gelatinous material and film conforms to the shape of the article and provide acoustic coupling between the active surface and the article without leaving a residue of the medium on the article.

GOVERNMENTAL INTEREST

The invention described herein may be manufactured, used and licensed byor for the Government for Governmental purposes without the payment tous of any royalties thereon.

This application is a continuation of application Ser. No. 07/935,698,filed Aug. 26, 1992, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to ultrasonic testing and moreparticularly to conformable acoustic couplers for use in ultrasonictesting.

A requirement exists to verify the existing chemical weapons stockpiledeclarations. However, a large part of the existing stockpiles arecontained within sealed munitions. Typically, chemical munitions can bedifferentiated from conventional munitions by external labeling.However, many of these munitions, particularly in third world countries,are not labeled. In addition, munitions might be improperly labeled toconceal their contents. Identification of the contents by conventionalanalytical methods would be dangerous and time consuming. Heretofore, nomethods are currently available to quickly and safely identify thecontents of sealed munitions. However, a number of existing technologiesappear to be adaptable to the task with some additional modifications.Ultrasonic technology is attractive because it is inherently safe,straightforward in application, has the potential to be miniaturized andis available at low cost. It is known in the art to conductnon-destructive testing of unknown materials within containers and thelike by means of ultrasonic test procedures. One of several possibleapproaches to ultrasonic interrogation is to measure the velocity ofsound in the various chemical agents. In this regard, U.S. Pat. Nos.2,527,986 and 2,398,701 pertain to such ultrasonic testing techniques.It is generally well known that sound travels through differentmaterials at different speeds. In the case of chemical verification, aspecific fill might be related to a specific velocity at a measuredtemperature. While different chemical agents can be correlated tospecific transmission velocities under controlled conditions, it hasbeen found that differentiation of munition fills on the basis ofvelocity measurements is not practical. One reason is that transmissionvelocities are highly dependent on temperature, which is easy to measurebut difficult to control in the field. In addition, the physicalcondition and purity of the chemical fills is known to varysignificantly, and the variations encountered in the measurement oftransmission velocities does not allow for reasonable statisticalcorrelations. Also, the speed of transmission is similar in a number ofunrelated materials and is not sufficiently unique for the intendedpurpose of verification of chemical stockpiles. In addition, severalother practical constraints beyond temperature and fill purity requireattention. The physical condition of the fills and the internalstructural configurations vary, sometimes significantly, in munitions ofsimilar type. In the case of filled munitions, information on theinternal structure or condition is often not known. As an example, theburster core within the center of the munition can be short, long or ofsome intermediate length. It may be empty, packed with explosives, orfilled with small explosive canisters. It is apparent that this highdegree of variability among like munitions must be taken intoconsideration. Another approach to ultrasonic interrogation is thestanding wave or resonance approach. In this regard see U.S. Pat. Nos.4,215,583 and 3,861,200. Variations of resonant ultrasound have beenused to test for structural integrity or homogeneity of variousmaterials. In this method a structure is stimulated to induce one ormore resonant frequencies and the frequency is compared with a referencespectrum for similarity. In the case of the present invention, themunitions would serve as the test structure and behave as an integralsystem. For a discussion of this method, one may see U.S. Pat. No.3,595,069 and Postany, G.J., Influence of the Pulser on the UltrasonicSpectrum: The Results of an Experiment"; Materials Evaluation, Mar.1965, pgs. 417-419. When stimulated with broadband excitation, thosematerials with a higher resonance transmission will tend to dominate thefrequency spectrum. In the testing of munitions, frequency transmissionis favored by the shell or container casing as opposed to the fill.Similar shells or containers would tend to resonate in a similar manner,and the damping effects of the fill would become a secondary resonanceeffect. Different fills would have different damping factors, and thuscan be differentiated. This approach is highly desirable for structuralanalysis of the munitions, but less so for determination of the contentsof the fill because the fill level is observed to have a large effect onthe frequency signature. In addition, the method in which the munitionis supported and the position of the transducer both have an observableand sometimes unpredictable effect on the frequency signature. Suchchanges in the frequency signature make correlations with stored spectradifficult.

A third method of ultrasonic interrogation, the pulse-echo methodappears to be somewhat more forgiving in subtle variations found in theresonance methods. One may see U.S. Pat. No. 3,595,069 and Kline, R.A.Measurement of Attenuation and Dispersion Using an UltrasonicSpectroscopy Technique". J. Acoust. Soc. Am. 76(2), Aug. 1985, pgs498-504, both of which are incorporated herein by references.

By this means, an acoustic transducer generates an acoustic pulsethrough an active surface which is thereby imparted to an article to betested. The pulsed article produces a reverberation which is received bya sensor, usually a piezoelectric device. The sensor then produces asignal which is measured by an analysis device such as a computer. Whilesuch ultrasonic testing is very frequently used in articles havingsubstantially flat surfaces, it has been a problem in the art toultrasonically test curved surfaces. This is because it has beendifficult to achieve a good acoustic energy coupling between the activesurface of the transducer and the curved article to be tested. It isknown in the art that when performing ultrasonic testing, close couplingshould be attained between the active surface of a transducer and thearticle to be tested. Acoustic coupling to non-flat surfaces such asammunition shells, storage tanks, etc., require coupling that easilyconforms to its surface. One attempt at solving this problem has been touse an aqueous interface or grease as a coupling medium between theactive surface and the article to be tested. In another attempt, plasticmaterials such as putty or modeling clay have been used. These haveproved to be unsatisfactory since such materials leave residues whichmay be corrosive or absorb chemical agents. A method of close couplingby applying a fluid coupling layer is suggested by Pedrix, M., et al,Acoustic Independence Measurement by Reflection of Ultrasonic Impulse ona Specimen through a Coupling Layer", Transactions on Sonics andUltrasonics, Vol. SU-28, No. 6, Nov. 1981. Coupling fluids are a problemwhen testing munitions since they tend to leave a residue and react withor absorb into the shell container.

For ease of operation and speed in testing a large variety of sizes andtypes of articles, a material is needed which does not leave a residueon the article to be tested. It also must be capable of functioning overa wide temperature range while being non-reactive, non-absorbing and notrequire subsequent cleaning of the tested article.

It is therefore an object of the invention to provide a conformableacoustic coupler for ultrasonically testing an article which quickly andeasily conforms to a variety of non-flat surfaces and permits quick andefficient testing over a wide range of ambient conditions. Anotherobject of the invention is to provide such a conformable acousticcoupler which does not leave a residue on the article which must beremoved and where the coupler is non-reactive with the article.

These and other objects will be in part described and in part apparentfrom a consideration of the detailed description of the preferredembodiment.

SUMMARY OF THE INVENTION

The invention provides a conformable acoustic coupler for ultrasonicallytesting an article. The coupler includes an ultrasonic transducer havingan active surface, which surface is capable of transmitting anultrasonic energy pulse therethrough, said transducer comprisingultrasonic pulse generating means capable of applying an ultrasonicenergy pulse by said active surface to an article; and ultrasonic sensormeans capable of receiving and measuring an ultrasonic echo from saidarticle in response to said applied ultrasonic energy pulse; and meansfor acoustically mating said article to the active surface of saidtransducer, said acoustic mating means comprising a layer of agelatinous material on said active surface and a flexible film on saidgelatinous material layer, which film substantially does not react withand substantially does not absorb said gelatinous material; wherein saidacoustic mating means is capable of conforming to the shape of at leastpart of said article.

The invention also provides a method for ultrasonically testing anarticle. The method includes the steps of providing an acoustic coupler,said acoustic coupler comprising an ultrasonic transducer having anactive surface, which surface is capable of transmitting an ultrasonicenergy pulse therethrough, said transducer comprising ultrasonic pulsegenerating means capable of applying an ultrasonic energy pulse by saidactive surface to an article; and ultrasonic sensor means capable ofreceiving and measuring an ultrasonic echo produced by said article inresponse to said applied ultrasonic energy pulse; and means foracoustically mating said article to the active surface of saidtransducer, said acoustic mating means comprising a layer of agelatinous material on said active surface and a flexible film on saidgelatinous material layer, which film substantially does not react withand substantially does not absorb said gelatinous material; wherein saidacoustic mating means is capable of conforming to the shape of at leastpart of said article; and juxtapositioning an article to be tested withsaid acoustic mating means such that said acoustic mating means conformsto the shape of at least part of said article; and applying anultrasonic energy pulse to said article through said active surface andsaid acoustic mating means; and receiving and measuring the ultrasonicecho produced by said article in response to said applied ultrasonicenergy pulse.

The invention further provides a method for ultrasonically testing amunitions article. The method includes the steps of providing anacoustic coupler, said acoustic coupler comprising an ultrasonictransducer having an active surface, which surface is capable oftransmitting an ultrasonic energy pulse therethrough, said transducercomprising ultrasonic pulse generating means capable of applying anultrasonic energy pulse by said active surface to a munitions article;and ultrasonic sensor means capable of receiving and measuring anultrasonic echo produced by said munitions article in response to saidapplied ultrasonic energy pulse; and means for acoustically mating saidmunitions article to the active surface of said transducer, saidacoustic mating means comprising a layer of a gelatinous material onsaid active surface and a flexible film on said gelatinous materiallayer, which film substantially does not react with and substantiallydoes not absorb said gelatinous material; wherein said acoustic matingmeans is capable of conforming to the shape of at least part of saidmunitions article; and juxtapositioning a munitions article to be testedwith said acoustic mating means such that said acoustic mating meansconforms to the shape of at least part of said munitions article; andapplying an ultrasonic energy pulse to said munitions article throughsaid active surface and said acoustic mating means; and receiving andmeasuring the ultrasonic echo produced by said munitions article inresponse to said applied ultrasonic energy pulse.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an embodiment of the conformableacoustic coupler for ultrasonically testing according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the practice of the present invention, an acoustic transducergenerates an acoustic pulse through an active surface which is therebyimparted to an article to be tested. The pulsed article produces areverberation which is received by a sensor which is a piezoelectriccrystal, and preferably the same device which generated the pulse. Thesensor then produces a signal which is measured by an analysis devicesuch as a spectrum analyzer and computer. In this approach, a broadbandfrequency is pulsed through a munitions article. As the pulse echoesback and forth between the walls of the munitions and through the fill,certain frequencies are attenuated more than others. The attenuationwould be dependent primarily upon the composition of the fill and to alesser degree on the geometry and composition of the shell or container.Since a single pulse is used to create the spectrum, a Fouriertransformation is the ideal treatment to reconstruct the spectralfingerprint. The broadband fingerprint would also be more suitable forstatistical correlations used in pattern matching algorithms since it isprimarily formed by the attenuating effects of the fill rather than thesecondary effects of damping on the resonance of the structure.

The preferred embodiment of the invention may be seen with reference todrawing FIG. 1. The testing device comprises a piezoelectric transducer2, such as a Panametrics 100 kHz commercially available as a PanametricsHigh Voltage Pulser model 5058 which comprises a piezoelectric crystal.The transducer has a BNC connector 4, which connects the transducer tothe pulser and data collection means which is preferably a HewlettPackard Spectrum Analyzer model 3567A for acquiring a frequency spectrumfrom 0 to 104 kHz and an industrial grade IBM PC compatible computer fordata collection, storage and comparison. The transducer has an activesurface 6 through which the ultrasonic pulses and echoes are transmittedto and acquired from the article to be tested. Applied to the activesurface of the transducer is a medium for closely coupling thetransducer to the shape of the article to be tested. The mediumcomprises a layer of a gelatinous material 8, such as petroleum jelly,silicone grease, water based jellies such as K-Y Jelly available fromJohnson & Johnson, and food gels such as Jell-O and honey. Putties andclays can also be used, however, these are less preferred. In thepreferred embodiment the gelatinous material layer has a thickness offrom about 0.125 to about 0.25 inch. This gelatinous materialestablishes good acoustic contact with the active surface of thetransducer. Encircling the gelatinous material is a thin film 10 whichserves to separate the gelatinous material from direct contact with thearticle to be tested. However, it is thin enough that it establishesgood acoustic contact with the gelatinous material, and hence the activesurface of the transducer and also conforms to the shape of the articleto be tested while also establishing good acoustic contact with thearticle. In the preferred embodiment, the flexible film comprises amaterial such as polyvinylidene chloride, commercially available assaran wrap, polyvinylidene fluoride, commercially available as Tedar,polyethylene, and polyethylene terephthalate. The film preferably has athickness of from about 0.5 to about 1.0 mil. The thin film can be heldto the transducer by a retaining belt 12 which can be a metal or rubberband. For testing, one merely places the article to be tested on theouter surface of the thin film. This configuration conforms to thesurface of the article to be tested and gives excellent acousticcoupling of the article to the transducer. This also allows rapidmovement from article to article during testing with no need to cleanthe article of gelatinous material after testing.

The following non-limiting examples serve to illustrate the invention.

EXAMPLE 1

A munitions shell is subjected to a single half wave electrical pulsethrough a single piezoelectric crystal transducer. The transducer servesas the signal source as well as the receiver of a pulse-echo. Acommercially available Panametrics High Voltage Pulser model 5058 with aPanametrics 100 kHz transducer is used as the source of excitation. AHewlett Packard Spectrum Analyzer model 3567A is used to acquire thefrequency spectrum from 0 to 104 kHz. An industrial grade IBM PCcompatible computer is used for data analysis and to acquire a frequencysignature. The shell acts as a resonant cavity with a large number ofcomplex resonant modes. When an impulse of energy is applied, a widerange of frequencies are present and decay rapidly. These variousfrequencies undergo different degrees of attenuation or resonance aswell as phase modulation depending on the composition of the fill.

In the performance of a test, an operator positions and holds thetransducer against the munition. In order to obtain enhanced coupling ofthe munition to the transducer, a thick film of K-Y Jelly, commerciallyavailable from Johnson and Johnson is applied to the active surface ofthe transducer. A film of saran wrap encircles the K-Y Jelly to separateit from the munition to be tested. A pulse signal is applied to themunition, pulse echo data is collected and the operator can either savethe data and compare the data either manually or automatically to alibrary reference spectra, an average spectra or to the data from anyother munition. Data is collected as a power spectrum over the frequencyrange 0 to 104 kHz where each power spectrum represents an average often (10) individual spectra.

EXAMPLE 2

Two 155 mm shells are filled wit air, water, ethylene glycol, sand andseveral aqueous suspensions, and then tested with the apparatusdescribed in Example 1. In all cases, each material gives a distinctivespectrum and identification of the contents is easily accomplished. Inaddition, fine degrees of distinction can often be made between similarmaterials. The signature of a solid suspension can be differentiatedfrom its unsuspended counterpart.

EXAMPLE 3

A series of 8 inch conventional HE (TNT) rounds, 155 mm conventionalComp B rounds, 155 mm VX rounds, 105 m GB rounds, 155 mm GB rounds and 1ton HD containers are tested using the apparatus described in Example 1and frequency signatures obtained. It is observed that variations intemperature have little effect on the frequency spectrum unless a phasechange occurs over the measured range. In addition, it is found that thefrequency spectrum is not affected by the general position of thetransducer, except when the transducer is placed at either extreme endof the munition or above the fill line. It is further observed that thefill level does not have an appreciable effect on the frequencysignature except when the container is almost empty. It is found thatone can establish the similarity and identity of contents of sealedcontainers on the basis of similarities of the reconstructed spectralsignatures.

These tests demonstrate that it is possible to rapidly field check alarge number of sealed munitions in a non-intrusive and non-destructivemanner. Analysis indicates a high correlation between similar rounds andrather low correlations between different chemical agents.

What is claimed is:
 1. A conformable acoustic coupler for ultrasonicallytesting an article, said coupler comprising:a) an ultrasonic transducerhaving an active surface, which surface is capable of transmitting anultrasonic energy pulse therethrough, said transducer comprisingultrasonic pulse generating means capable of applying an ultrasonicenergy pulse by said active surface to an article; and ultrasonic sensormeans capable of receiving and measuring an ultrasonic echo from saidarticle in response to said applied ultrasonic energy pulse; and b)means for acoustically mating said article to the active surface of saidtransducer, said acoustic mating means comprising a layer of agelatinous material on said active surface and a flexible film on saidgelatinous material layer, which film substantially does not react withand substantially does not absorb said gelatinous material; wherein saidacoustic mating means is capable of conforming to the shape of at leastpart said article.
 2. The coupler of claim 1 wherein said flexible filmcomprises a material selected from the group consisting ofpolyvinylidene chloride, polyvinylidene fluoride, polyethylene, andpolyethylene terephthalate.
 3. The coupler of claim 1 wherein saidflexible film has a thickness of from about 0.5 to about 1.0 mil.
 4. Thecoupler of claim 1 wherein said gelatinous material layer comprises amaterial selected from the group consisting of petroleum jelly, siliconegrease, water based jellies and food gels.
 5. The coupler of claim 1wherein said gelatinous material layer has a thickness of from about0.125 to about 0.25 inch.
 6. The coupler of claim 1 wherein saidtransducer comprises a piezoelectric crystal.
 7. The coupler of claim 1wherein said transducer generates an ultrasonic energy pulse having afrequency in the range of more than 0 to about 104 kHz.
 8. A method forultrasonically testing an article comprising the steps of:i) providingan acoustic coupler, said acoustic coupler comprising:a) an ultrasonictransducer having an active surface, which surface is capable oftransmitting an ultrasonic energy pulse therethrough, said transducercomprising ultrasonic pulse generating means capable of applying anultrasonic energy pulse by said active surface to an article; andultrasonic sensor means capable of receiving and measuring an ultrasonicecho produced by said article in response to said applied ultrasonicenergy pulse; and b) means for acoustically mating said article to theactive surface of said transducer, said acoustic mating means comprisinga layer of a gelatinous material on said active surface and a flexiblefilm on said gelatinous material layer, which film substantially doesnot react with and substantially does not absorb said gelatinousmaterial; wherein said acoustic mating means is capable of conforming tothe shape of at least part said article; and ii) juxtapositioning anarticle to be tested with said acoustic mating means such that saidacoustic mating means conforms to the shape of at least part of saidarticle; andiii) applying an ultrasonic energy pulse to said articlethrough said active surface and said acoustic mating means; and iv)receiving and measuring the ultrasonic echo produced by said article inresponse to said applied ultrasonic energy pulse.
 9. The method of claim8 wherein said flexible film comprises a material selected from thegroup consisting of polyvinylidene chloride, polyvinylidene fluoride,polyethylene, and polyethylene terephthalate.
 10. The method of claim 8wherein said flexible film has a thickness of from about 0.5 to about1.0 mil.
 11. The method of claim 8 wherein said gelatinous materiallayer comprises a material selected from the group consisting ofpetroleum jelly, silicone grease, water based jellies and food gels. 12.The method of claim 8 wherein said gelatinous material layer has athickness of from about 0.125 to about 0.25 inch.
 13. The method ofclaim 8 wherein said transducer comprises a piezoelectric crystal. 14.The method of claim 10 wherein said transducer generates an ultrasonicenergy pulse has a frequency in the range of more than 0 to about 104kHz.
 15. A method for ultrasonically testing a munitions articlecomprising the steps of:i) providing an acoustic coupler, said acousticcoupler comprising:a) an ultrasonic transducer having an active surface,which surface is capable of transmitting an ultrasonic energy pulsetherethrough, said transducer comprising ultrasonic pulse generatingmeans capable of applying an ultrasonic energy pulse by said activesurface to a munitions article; and ultrasonic sensor means capable ofreceiving and measuring an ultrasonic echo produced by said munitionsarticle in response to said applied ultrasonic energy pulse; and b)means for acoustically mating said munitions article to the activesurface of said transducer, said acoustic mating means comprising alayer of a gelatinous material on said active surface and a flexiblefilm on said gelatinous material layer, which film substantially doesnot react with and substantially does not absorb said gelatinousmaterial; wherein said acoustic mating means is capable of conforming tothe shape of at least part said munitions article; and ii)juxtapositioning a munitions article to be tested with said acousticmating means such that said acoustic mating means conforms to the shapeof at least part of said munitions article; and iii) applying anultrasonic energy pulse to said munitions article through said activesurface and said acoustic mating means; and iv) receiving and measuringthe ultrasonic echo produced by said munitions article in response tosaid applied ultrasonic energy pulse.
 16. The method of claim 15 whereinsaid flexible film comprises a material selected from the groupconsisting of polyvinylidene chloride, polyvinylidene fluoride,polyethylene, and polyethylene terephthalate.
 17. The method of claim 15wherein said flexible film has a thickness of from about 0.5 to about1.0 mil.
 18. The method of claim 15 wherein said gelatinous materiallayer comprises a material selected from the group consisting ofpetroleum jelly, silicone grease, water based jellies and food gels. 19.The method of claim 15 wherein said gelatinous material layer has athickness of from about 0.125 to about 0.25 inch.
 20. The method ofclaim 15 wherein said transducer comprises a piezoelectric crystal. 21.The method of claim 15 wherein said transducer generates an ultrasonicenergy pulse having a frequency in the range of more than 0 to about 104kHz.
 22. The method of claim 15 wherein said munitions article comprisesan artillery shell.
 23. The method of claim 15 wherein said munitionsarticle comprises a chemical warfare agent.
 24. The method of claim 22wherein said munitions article comprises a chemical warfare agent.